The relationship Between Earliness and Vigour in Wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) is the most widespread winter crop sown in
south-eastern Australia, where it is often grown under water-limited
conditions. Grain yield potential is increased when cultivars have phasic
development adapted to the environment in which they are grown, and are
capable of accumulating biomass whilst maintaining water-use efficiency to
achieve a high harvest index. There has been some anecdotal evidence that
suggests rate of development and the accumulation of biomass may be
associated, with reports that early-maturing cultivars grow faster than latermaturing cultivars. A series of field and pot experiments were conducted
during 2008-11 to investigate the association between rate of phasic
development and plant growth in wheat in southern New South Wales. The
experiments presented in this thesis show that later maturing, vernalisationresponsive genotypes accumulate biomass slower than early-maturing genotypes. Photoperiod and vernalisation genes have a significant influence on developmental rate. These genes are described as regulatory genes and have been shown to influence plant traits in addition to their effect on
phenology. The influence of these genes on early growth in wheat is
unknown, though information pertaining to these effects would make a
significant contribution to breeder’s ability to ‘design’ genotypes suited to
various environments and farming systems. The investigations undertaken
were divided into two main areas: (i) the physiological association between
rate of plant development and early growth; and (ii) the role of vernalisation
and photoperiod genes in regulating plant development and growth.

A positive correlation between shoot and root biomass was established for a
group of cultivars, indicating that differences in growth rate were not due to
genotypic variation in the distribution of biomass. Using lines from a
doubled-haploid population from the cross Janz/Diamondbird, the effect of
Ppd-B1, Ppd-D1, Vrn-A1, Vrn-B1 and Vrn-D1 on anthesis date, plant stature
and growth were measured. The effect of these genes was to account for
75% of the genotypic variance in anthesis date and 85% of the genotypic
variance in plant stature in the given population. Presence of the winter
allele at either Vrn-A1 or Vrn-B1 delayed anthesis and reduced plant stature.
Genotypes with winter alleles at all three VRN1 loci (Vrn-A1v + Vrn-B1v +
Vrn-D1v), classified as winter types, had the largest delay in anthesis date
and the lowest plant stature scores. Presence of the winter allele Vrn-B1v
consistently reduced biomass and slowed crop growth rate compared with
the spring allele Vrn-B1a. In one experiment Vrn-A1v suppressed growth
relative to Vrn-A1a. It is suggested that the effect of VRN1 genes on plant
growth is a pleiotropic effect of these genes. The effect of VRN1 genes on
plant growth is not consistent with their effect on development and plant
stature. Whilst anthesis date was further delayed and plant stature reduced
by sequential substitution of spring alleles with winter alleles, the
suppression of growth reported for Vrn-B1v (and Vrn-A1v) was not
enhanced by the presence of winter alleles at the other VRN1 loci. These
differences in growth rate are not evident in controlled glasshouse
experiments characterized by relatively high growing temperatures or in a
field experiment where sowing was significantly delayed and temperatures
were not cold enough to saturate the vernalisation response of winter
genotypes.

This study has attributed the association observed between vigour and
earliness in wheat to a pleiotropic effect of the VRN1 genes rather than a
direct association with development per se. These genes delay anthesis and
reduce plant stature in vernalisation-responsive genotypes, and when
expressed under cold temperatures, they are capable of suppressing plant
growth